1. Field of Invention
This invention relates in general to satellite navigation systems and in particular to mitigating the effects of cross correlation in Global Positioning System (GPS) receivers.
2. Related Art
Cross correlation between a strong received satellite signal with the C/A (clear/acquisition) code of a weak satellite signal may prevent the weak satellite signal from being acquired/tracked. This problem may occur where the weak satellite signal is blocked by physical obstruction or in an indoor environment where the satellite signal is severely degraded. It is desirable to mitigate cross correlation in order to acquire/track the weak signal.
Past methods of cross correlation mitigation have included the classic post correlation subtraction method or the subspace projection method. The post correlation subtraction method typically relies on computing the cross correlation signature of the weak satellite C/A with a generated signal representing the strong satellite to be mitigated. The generated signal uses the carrier phase, carrier frequency, and code phase obtained from tracking of the strong satellite to estimate the received strong satellite signal. The correlation signature is then scaled by the estimated strong satellite amplitude and subtracted from the correlation of the received signal with the weak satellite C/A code. Alternatively, in the subspace projection method the correlation signature is compared against a threshold and the weak satellite C/A code is modified for received signal correlation so as to reduce the cross correlation of the strong satellite signal with the modified C/A and to enable the weak satellite signal to be acquired/tracked.
However, both the post correlation subtraction method and the subspace projection method suffer from poor mitigation performance in dynamic environments. Correlation signature is sensitive to the amplitude estimate error and phase estimate error of the strong satellite signal. Estimate of the satellite parameters may not be accurate due to Doppler and fading that often occurs in dynamic environments. In the subspace projection method, a different C/A code would have to be derived for each code phase position used in the correlation signature. Integration of the modified C/A code into the correlation with the received signal is also complicated to implement due to the code phase dependency of the correlation signature.
Therefore, there is a need for a system and method capable of mitigating the effects of cross correlation in a GPS receiver.